According to the unified scheme of active galactic nuclei (AGNs), a
blazar is considered to be a radio-loud AGN that displays highly variable,
beamed, non-thermal emission covering a broad wavelength range from radio
to \(\gamma \) -ray energies [1]}. The properties of the blazar
non-thermal emission suggest a relativistic origin in a jet oriented at
a small angle to our line of sight [2]}. The blazar
class encompasses BL Lacertae (BL Lac) objects and flat-spectrum radio quasars
(FSRQs). The most striking difference between the two lies in their optical spectra, with the former showing
weak or no emission or absorption lines [3]} and the latter strong broad emission
lines. One difficulty in the study of BL Lacs is indeed that in many cases
it is not possible to measure any redshift because of a lack of features in
their optical spectra [4]}. Blazars, and in
particular BL Lacs, are very strong \(\gamma \) -ray emitters, being the most numerous extragalactic sources at high energies
[5]}, [6]}, [7]}.
